160 THE OLD RED SANDSTONE. 
Holoptychius of the Coal Measures, but the contrast in size 
was somewhat less marked. One of the most singularly 
formed bones of the formation will be found, I doubt not, 
when perfect specimens of the upper part of the creature 
shall be procured, to have belonged to the Holoptychius. \t 
is a huge ichthyodorulite, formed, box-like, of four nearly 
rectangular planes, terminating in a point, and ornamented 
on two of the sides by what, in a work of art, the reader 
would at once term a species of Chinese fretwork. Along 
the centre there runs a line of lozenges, slightly truncated 
where they unite, just as, in plants that exhibit the cellular 
texture, the lozenge-shaped cells may be said to be truncated. 
At the sides of the central line, there run lines of half loz- 
enges, which occupy the space to the edges. Each lozenge 
is marked by lines parallel to the lines which describe it, 
somewhat in the manner of the plates of the tortoise. The 
centre of each is thickly tubercled ; and what seems to have 
been the anterior plane of the ichthyodorulite is thickly tuber- 
cled also, both in the style of the occipital plates and jaws of 
the Holoptychius. This curious bone, which seems to have 
been either hollow inside, or, what is more probable, filled 
with cartilage, measures, in some of the larger specimens, 
an inch and a half across at the base on its broader planes, 
and rather more than half an inch on its two narrower 
ones.* 
Geologists have still a great deal to learn regarding the 
contemporaries of the Holoptychius nobilissimus. The 
lower portion of that upper formation to which it more 
* This bone has been since assigned by Agassiz to a new genus, of 
which no other fragments have yet been found, but which has been 
named provisionally Placothorax paradoxus. 
